Centrifugal casting is a popular art that has been in existence for many years, especially in the jewelry industry, where, for example, by using the lost wax process, a simple mold can be made in plaster of Paris, or suitable gypsum products of industrial type. An ingot of precious metal or otherwise is placed in a muffle on one end of a horizontal rod mounted on the upper end of a vertical shaft which is rotated by any suitable motor. The muffle is usually electrically heated to melt the ingot and, when molten, it is introduced to the cavity in the mold and centrifical action insures full projection of the molten material into the mold and, when done expertly, no cavities or holes due to residual air occur in the finished product. The opposite end of the horizontal rod usually supports a counterweight to avoid the need of securely anchoring the rotatable shaft by suitably embedding the lower end thereof in a mass of heavy material such as Portland cement or otherwise.
Centrifugal casting such as that broadly described above also is employed in the dental art for purposes of making crowns, inlays, and certain other related types of metal items employed in restorative dentistry, the lost wax process also frequently being used to make the necessary mold in which cavities of desired shapes are formed. One simple form of casting metallic dental material to form a molded object is the subject matter of prior U.S. Pat. No. 1,563,151 to Booth, dated Nov. 24, 1925 and a later, more sophisticated machine of this broad type comprises the subject matter of prior U.S. Pat. No. 2,235,443 to Steinbock et al, dated Mar. 18, 1941, both of the aforementioned patents revolving the muffles in a vertical plane about a horizontal shaft and in both of these counterweights are employed on the end the rotatable arm or bar which is opposite the muffle.
Other forms of centrifugal casting machines for making cast dental objects and in which the arms that support the muffles and counter balances are disposed for operation about a vertical axis in a horizontal plane comprise the subject of prior U.S. Pat. Nos. 2,749,585 to Brosen, dated June 12, 1956; 4,077,060 to Halatek, dated Mar. 7, 1978; and 4,134,445 to Goodrich et al, dated Jan. 16, 1979, the examples of operation shown therein primarily employing counterweights in suitable arrangement so that vibration is minimized and comprise so called bench type centrifugal casting machines which do not require substantial anchoring because of the muffles and counterweights counterbalancing each other.
Still another prior U.S. Pat. No. 4,280,551 to Ohara, dated July 28, 1981 illustrates a somewhat more sophisticated type of centrifugal casting apparatus for dentistry in which the rotatable shaft for the transverse arm on one end thereof which carries the muffle and counterweight is disposed at an angle of substantially 45.degree. to the horizontal.
The present invention also pertains to a centrifugal casting furnace, especially for dental purposes, which includes a transverse arm carried by the upper end of a vertical rotatable shaft and an electrically heated muffle is mounted adjacent one end of said arm, while a counterweight is carried by the opposite end of the arm, and is adjustable in certain ways that are distinct from the prior art and the invention also includes other beneficial and meritorious characteristics that likewise are not found in the prior art and especially the type of art referred to above. The present invention is especially directed to centrifugal ceramic casting furnaces for casting glass dental prosthetic parts. Details of such innovations and characteristics are set forth below.